1. Field of the Invention
The present invention relates to a disc player for playing a recording disc such as a digital audio disc or a video disc.
2. Description of Background Information
Disc players are structured such that an optical beam is used to trace a recording tract constituted by depressions called pits formed in the recording surface of the disc, and a recording signal is read-out by detecting the light from the recording surface of the disc. For such operations, it is necessary to use a focus servo system for converging the light beam onto the surface of the disc to form a light spot for reading information (information reading point) and a tracking servo system for moving the information reading point so that it follows the recording track accurately.
For the focus servo system, systems according to a so-called astigmatic method are known. Such systems are arranged such that the light beam is converged, by using a cylindrical lens for example, as a focal line in the horizontal direction at one of two separate points on the light path of the light beam. The light beam is also converged as a focal line in the vertical direction at the other of the two points. A light sensor of a quadrant type is disposed at a middle position of the two points, to generate an error signal.
For the tracking servo system, systems according to a so-called three-beam method are known. Such systems are arranged such that three beams including a main beam for reading information and two sub-beams for detecting tracking errors disposed on both sides of the main beam are prepared and arranged so that a line passing through the center of the three beams forms a predetermined off-set angle with respect to the direction of the track, and an error signal is generated from a difference between the amount of the two sub-beams reflected by the recording surface of the disc.
With this type of tracking servo systems, if there is a defect called a splash defect such as a flaw of the aluminium layer evaporated on the recording pits, the defect is read by the sub-beam, and a pulse-like noise will be generated in the tracking error signal. In such an event, the noise in the tracking error signal in turn causes a fault called a track jumping by which the information reading light spot erroneously jumps to another track. In the focus servo systems, if there is a defect called black spot defect such as a stain on the recording surface of the disc, a noise will be generated in the focus error signal. In such an event, the control operation of the focus servo system is disturbed, and the sub-beams of the tracking servo system are adversely affected by such a disturbance. This also will result in the track jumping fault.
As described above, faults of the disc do not occur uniformly, and it has been very difficult to prevent faults caused by a defect of the disc such as the track jumping fault.
More specifically, in order to cope with faults cause by the defect of the disc it is effective to lower the loop gain of the servo loop so as to reduce the influence of noises. However if a vibration is applied to the player from outside, the relative distance between the disc and the objective lens is varied by the vibration, and this also will cause the track jumping fault. For the track jumping fault caused by external vibrations, it is effective to increase the loop gain of the servo loop, so as to improve the response of the servo loop against the change in the relative position. It then follows that, a measure to counter the defect of the disc will deteriorate the stability of the player against vibrations. Thus it has been very difficult to prevent faults by the defect of the disc and faults by external vibrations at the same time.